1. Field of the Invention:
The present invention relates generally to electronic circuits, and more particularly to monolithic integrated operational amplifier circuits.
2. Description of the Prior Art:
Operational amplifiers are quite commonly used in electronic circuit design. The circuits considered herein to be operational amplifiers generally are monolithic integrated circuits which implement a high gain amplifier having differential inputs. Variations are sometimes implemented, such as single input operational amplifiers having a second input tied internally to ground.
Operational amplifiers are available in a number of different designs which have been optimized for particular uses. Operational amplifiers emphasizing features such as high or lower power, fast response time, large bandwidth, or general purpose use, are available.
One useful design for an operational amplifier circuit is one that provides a low power drain combined with a high slew rate and good precision. The high slew rate would preferably be greater than approximately 40 V/microsecond, and provide a minimum amount of overshoot during the settling time.
Stability requirements for such a fast operational amplifier imply that a large phase margin, preferably greater than 60.degree., and large bandwidth be provided. In order to provide a good phase margin, it is preferable to avoid the use of Darlington transistor pairs in the output stage. A simple output stage is therefore preferred in fast operational amplifiers, but the constant current needed to bias such a simple output stage is relatively high. The bias current must be high enough so that the operational amplifier performs properly in the worst case, which occurs when high output currents are being sourced and a rising edge occurs. The relatively high bias currents needed to properly bias the output stage against this worst case are inconsistent with the need for a low power operational amplifier.
It would therefore be desirable to provide an operational amplifier which combines a high slew rate combined with low bias power requirements.